1. Field of the Invention
The present invention relates to a laser scanning apparatus that finds application in fields involving image formation.
2. Description of the Prior Art
Image forming apparatuses such as printers and copiers incorporate a laser scanning apparatus that forms a latent image of an object image on a photoconductive member by scanning the photoconductive member with laser light that converges thereon. A laser scanning apparatus is composed of a laser light source that emits laser light, a deflector that deflects the laser light from the laser light source in a main scanning direction in which to scan the photoconductive member, and an optical system that makes the laser light converge on the photoconductive member. The optical system for making the laser light converge on the photoconductive member is typically composed of two parts, namely a pre-deflector optical system disposed between the laser light source and the deflector to control the convergence of the laser light principally in the sub scanning direction (the direction perpendicular to the main scanning direction) and a post-deflector optical system disposed between the deflector and the photoconductive member to control the convergence of the laser light in both the main and sub scanning direction.
The post-deflector optical system needs to be designed to permit the laser light to form an extremely small spot at every position at which it is incident on the photoconductive member (i.e., all over the scanned region) despite the fact that the optical path length to the photoconductive member and the angle of incidence with respect to the photoconductive member vary according to the position at which the laser light is incident on the photoconductive member. Thus, building the post-deflector optical system by using common spherical optical elements tends to result in making it unduly complicated and large.
To overcome this, proposals have been made to reduce the number of needed optical elements by using aspherical or free-form-surfaced optical elements. For example, Japanese Patent Application Laid-Open No. H11-153764 proposes a construction in which a post-deflector optical system is provided with a free-form-surfaced mirror so that laser light is made to converge by that free-form-surfaced mirror alone. This helps simplify and miniaturize the construction.
However, even when a free-form-surfaced optical element is used, there is a limit to its aberration correction performance. Thus, it is difficult to achieve satisfactory convergence of laser light all over a scanned region by the use of a single free-form-surfaced mirror. The shape of the laser light spot tends to be deformed particularly at both ends of the scanned region, leading to poor image quality in peripheral portions of the formed image.